If you purcase parts and materials for a job bill it to that job in your accounting program. Any thing left over can be transfered to inventory later. Be careful of this. Some states will charge you a property tax on inventory on hand at the end of a year. Also a large inventory will have items that just occupy space for years if you let them. Any inventory has cost you money and will continue to cost storage space untill the item is used. If you chose to keep inventory be sure it is used regularly. How often depends on how much money you want to tie up is have items on a shelf.